HK Audio CDR 210 F - Manual (Page 9)

9
A
3.2 Midrange
A frequently occurring and often cited drawback
of line arrays is their sensitivity to wind. A look at
the construction of conventional line arrays reveals
that many do without a CD horn. Therefore, the
HF pattern of throw is increasingly constricted. It
is not uncommon for a radiation angle of an HF
unit nominally rated for 120° to come to just 50° at 16
kHz. A light wind from the side audibly carries away
the HF signal. In the case of conventional speaker
clusters this is not so significant because several
HF units are arrayed horizontally and these pick
up the slack to provide HF sound reinforcement.
In a line array, however, it is always just one HF unit
that is arrayed horizontally to target a predetermined
audience area!
A special speaker cover is located in front of the two
8" midrange speakers. Designed to shape the horn
contour with CD characteristic in front of the acoustic
lens, it is free of undesirable diffractive effects and
does not adulterate the sonic image. It does not
constrict the HF range and ensures greater stability
in the face of gusts of wind.
The sound of the 8" speakers emanates through
slots on the top and bottom of this cover. This
enhances the dynamic balance in the range of 130
to 900 Hz.
In addition, the cover creates a compression chamber
that behaves like an acoustical filter, suppressing
frequencies that lie above the rendered frequency
range. Rather than being dissipated, the energy is
displaced to a lower frequency range, thereby sub-
stantially increasing efficiency in the rendered fre-
quency range! Because the speaker now acts against
a limited volume of air, distortion decreases so that
very high levels can be achieved with little distortion,
much like in a horn.
3.3 The Subwoofer-
Truest Dynamic Response
A great drawback of conventional woofers is the
considerable moving mass of the speaker (general-
ly, 18"). As a result, low frequency reproduction is
marred by dynamic distortion.
There is only way to satisfy the desire for the driest,
cleanest possible low end response; that is to achieve
the best possible impulse behavior. By definition,
this requires a smaller speaker membrane with less
moving mass. The answer is a special 10" woofer.
Compared below is a typical 18"+12"+2" combina-
tion and the COHEDRA
® solution. Notably, the
speaker membrane's moving mass in the low and
middle frequency ranges is clearly reduced.
Superior impulse behavior is also evident in the 10"
speaker's transient response. In comparison with
an 18" woofer, the membrane of the 10" woofer is
able to move much faster because its transients are
shorter.
The blue curve shows the in theory ideal response
and the red curve indicates the 10" speaker's transient
response. The purple curve shows the transient re-
sponse of the 18" woofer. Measurements were taken
using a second order, 130Hz low-pass filter.
New filtering approaches were necessary to make
the most of the 10" woofer's capabilities, which is
why the housing also sports some special design
features. In combination with the bass reflex chan-
nel, the baffle boards form a horn, resulting in a
higher level in the low frequency range. The long,
lean chassis supports the formation of bass arrays
as well as ground coupling. In addition, extremely
compact clusters can be configured by setting en-
closures on end. Because the loudspeaker chassis is
tropic-proof, the enclosure can do without a level-
dampening front covering.
Figure 9: Front view of the COHEDRA®
CDR 208 mid/high enclosure
Figure 10: 8" speaker cover serving as a CD horn and
part of the compression chambe
Figure 12: Impulse response: ideal (blue), 10" (red), 18" (purple)
Figure 11: Comparison of moving masses
COHEDRA ® Technology